<PubML>® eBooks rely on your web browser's native bookmarking functionality. Every column of every section in your book is assigned a unique web address after factors like font size, typeface selection, alignment, and spacing have been considered.
This all works great until you display the eBook in an iframe—sort of a web page within a web page. <PubML>® eBooks embedded in a WordPress page or post are rendered in an iframe so they don't affect your browser's address bar (If they did, the bookmark would lead back to the appropriate page in the book but wouldn't display the book embedded within the post or page as the author intended). To avoid displaying bookmarking functionality in situations where it doesn't work, <PubML>® eBooks are "smart." They show or hide the bookmark ribbon based on how/where it's being displayed.